176 THE GREEK ASTRONOMY. 



successive revolutions. Thus its latitude, or dis- 

 tance from the equator, has a cycle different from 

 its revolution among the stars ; and its Nodes, or 

 the points where it cuts the equator, are perpe- 

 tually changing their position. In addition to this, 

 the moon's motion in her own path is not uniform ; 

 in the course of each lunation, she moves alter- 

 nately slower and quicker, passing gradually through 

 the intermediate degrees of velocity ; and goes 

 through the cycle of these changes in something 

 less than a month : this is called a revolution of 

 Anomaly. When the moon has gone through a 

 complete number of revolutions of Anomaly, and 

 has, in the same time, returned to the same posi- 

 tion with regard to the Sun, and also with regard 

 to her Nodes, her motions with respect to the sun 

 will thenceforth be the same as at the first, and 

 all the circumstances on which lunar eclipses de- 

 pend being the same, the eclipses will occur in 

 the same order. In 6585J days there are 239 

 revolutions of anomaly, 241 revolutions with regard 

 to one of the nodes, and, as we have said, 223 

 lunations or revolutions with regard to the sun. 

 Hence this period will bring about a succession of 

 the same lunar eclipses. 



If the Chaldeans observed the moon's motion 

 among the stars with any considerable accuracy, 

 so as to detect this period by that means, they 

 could hardly avoid discovering the anomaly or un- 

 equal motion of the moon ; for in every revolution, 



